SEDIMENTOLOGY AND DEPOSITIONAL SETTING OF THE BURGESS SHALE'S GREATER PHYLLOPOD BED

High resolution sedimentologic study of the Burgess Shale Formation's Greater Phyllopod bed (GPB), permits a reevaluation of both the mechanism for sediment emplacement and the environmental conditions that prevailed at the time. The geometry of the basin, the sedimentologic characteristics of the beds, as well as taphonomic trends in the fossil assemblages were utilized to critically assess the various depositional scenarios postulated for the Burgess Shale.

The majority of soft-bodied fossils are entombed within many of the mudstone beds at the GPB. These mudstones are interbedded with calcisiltite layers, which also contain soft-bodied fossils; however the quality of preservation is diminished in these coarser layers compared to the mudstones. The mudstone beds are most often massive, sharp based and genetically unrelated to the calcisiltites, and are not coupled, graded beds as previously reported. The variation and distribution of pyrite, from individually dispersed framboids to framboidal layers covering soft-bodied fossils, to concentrated clusters, all within the mudstone beds suggests that soft-bodied preservation (SBP) is not dependant on a particular redox state of the interstitial pore-water. The degree of SPB varies principally due to the grain size character of the host sediments.

Some of the GPB fossils appear to be in situ. However, the paucity of burrows and trackways from a community that was largely benthonic remains convincing evidence that the majority of organisms preserved at the GPB are allochthonous. Sediment patterns such as massive beds and high clay content are inconsistent with deposition via turbidity currents, and fluidized mud-flows respectively. The clinoform geometry of the basin is most consistent with transport of sediment off the escarpment perpendicular to the strike of the platform edge. We conclude that the most likely depositional scenario is that the siliciclastic muddy sediments and the majority of organisms were derived from the platform above the escarpment. The biota were likely carried over the muddy platform edge by storm generated back-currents where they subsequently settled out of the water column and were entombed in the muds that rained out around them¹.

¹ Initially proposed by: Aitken J and McIlreath I 1990. Geoscience Canada, 17:111-116.